CVD diamond for coating twist drills

ABSTRACT

The present invention enables the diamond coating of stationary elongate objects, such as twist drills, with a continuous uniform film without any motion of the twist drill due to the unexpected superb &#34;throwing power&#34; of a reactor disclosed herein. The CVD diamond reactor includes a vacuum chamber, inlet for feed hydrogen/hydrocarbon mixtures, and an outlet, in conventional fashion. The improvement for coating with CVD diamond the entire outer surface of at least a portion of a plurality of stationary elongate objects comprises disposed within said reactor, an elongate metal tube having a plurality of apertures for holding elongate objects disposed radially inwardly and having a cooling pipe in thermal contact with and disposed about the outside of said metal tube; and a filament running within said tube along its lengthwise extent and being in electrical connection with the source of voltage for heating said filament to a temperature adequate to initiate hydrocarbon disassociation, the portions of said elongate object within said tube surrounding said filament being heated thereby.

This is a division of U.S. patent application Ser. No. 07/816,411 filedDec. 30, 1991, abandoned which is a continuation of U.S. patentapplication Ser. No. 07/563,367, filed Aug. 7, 1990, now U.S. Pat. No.5,096,736.

BACKGROUND OF THE INVENTION

The present invention relates to diamond coating of workpieces and moreparticularly to an improved reactor design and process therefor.

Its hardness and thermal properties are but two of the characteristicsthat make diamond useful in a variety of industrial components.Initially, natural diamond was used in a variety of abrasiveapplications. With the ability to synthesize diamond by highpressure/high temperature (HP/HT) techniques utilizing acatalyst/sintering aid under conditions where diamond is the thermallystable carbon phase, a variety of additional products found favor in themarketplace. Polycrystalline diamond compacts, often supported on atungsten carbide support in cylindrical or annular form, extended theproduct line for diamond additionally. However, the requirement of highpressure and high temperature has been a limitation in productconfiguration, for example.

Recently, industrial effort directed toward the growth of diamond at lowpressures, where it is metastable, has increased dramatically. Althoughthe ability to produce diamond by low-pressure synthesis techniques hasbeen known for decades, drawbacks including extremely low growth ratesprevented wide commercial acceptance. Recent developments have led tohigher growth rates, thus spurring recent industrial interest in thefiled. Additionally, the discovery of an entirely new class of solids,known as "diamond like" carbons and hydrocarbons, is an outgrowth ofsuch recent work.

Low pressure growth of diamond has been dubbed "chemical vapordeposition" or "CVD" in the field. Two predominant CVD techniques havefound favor in the literature. One of these techniques involves the useof a dilute mixture of hydrocarbon gas (typically methane) and hydrogenwherein the hydrocarbon content usually is varied from about 0.1% to2.5% of the total volumetric flow. The gas is introduced via a quartztube located just above a hot tungsten filament which is electricallyheated to a temperature ranging from between about 1750° to 2400° C. Thegas mixture disassociates at the filament surface and diamonds arecondensed onto a heated substrate placed just below the hot tungstenfilament. The substrate is held in a resistance heated boat (oftenmolybdenum) and heated to a temperature in the region of about 500° to1100° C.

The second technique involves the imposition of a plasma discharge tothe foregoing filament process. The plasma discharge serves to increasethe nucleation density, growth rate, and it is believed to enhanceformation of diamond films as opposed to discrete diamond particles. Ofthe plasma systems that have been utilized in this area, there are threebasic systems. One is a microwave plasma system, the second is an RF(inductively or capacitively coupled) plasma system, and the third is ad. c. plasma system. The RF and microwave plasma systems utilizerelatively complex and expensive equipment which usually requirescomplex tuning or matching networks to electrically couple electricalenergy to the generated plasma. Additionally, the diamond growth rateoffered by these two systems can be quite modest.

Heretofore, CVD diamond has been coated onto tungsten carbide or othersubstrates to make cutting tool inserts (U.S. Pat. Nos. 4,707,384 and4,731,296) or co-deposited with boron or another element for makingsemiconductors (e.g. EP Pubilications Nos. 286,306 and 282,054). Incoating workpiece surfaces disposed opposite the heated filament (e.g.the back sides of twist drills), rotating the workpiece to ensurecomplete coverage is an extra process step to be avoided.

BROAD STATEMENT OF THE INVENTION

The present invention enables the diamond coating of stationary elongateobjects, such as twist drills, with a continuous uniform film withoutany motion of the twist drill due to the unexpected superb "throwingpower" of a reactor disclosed herein. The CVD diamond reactor includes avacuum chamber, inlet for feed hydrogen/hydrocarbon mixtures, and anoutlet, in conventional fashion. The improvement for coating with CVDdiamond comprises disposed within said reactor, an elongate metal tubehaving a plurality of apertures for holding elongate objects disposedradially inwardly and having a cooling pipe in thermal contact with anddisposed about the outside of said metal tube. A filament runs withinsaid tube along its lengthwise extent and is in electrical connectionwith a source of voltage for heating said filament to a temperatureadequate to initiate hydrocarbon disassociation. Portions of theelongate objects within the tube surrounding said filament are heatedthereby.

Another aspect of the present invention is a method for coating with CVDdiamond the entire outer surface of at least a portion of a plurality ofstationary elongate objects wherein a flow of hydrogen/hydrocarbonmixture is passed into a vacuum chamber of a diamond reactor held underCVD diamond-forming conditions wherein the mixture is disassociated fordiamond deposition/growth on a substrate disposed therein. Theimprovement in process comprises passing said feed hydrogen/hydrocarbonmixture through an elongate metal tube disposed within said reactorwhich tube has a plurality of apertures which hold elongate objectsdisposed radially inwardly and which has a cooling pipe in thermalcontact with and disposed about the outside of said metal tube. Afilament running within said tube along its lengthwise extent and whichis in electrical connection with a source of voltage for its heating isheated to a temperature adequate to initiate hydrocarbon disassociation.The heated filament heats the elongate objects and cooling fluid passingthrough the cooling pipe is adjusted to maintain the elongate objects ata CVD diamond-forming temperature.

Advantages of the present invention include a reactor design and processthat enables the coating of the entire surface of elongate objectswithin a CVD reactor without having to rotate the objects to accomplishsuch coating. Another advantage is the ability to coat a plurality ofsuch stationary elongate objects. These and other advantages will bereadily apparent based upon the disclosure contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional elevational view of the reactor componentsenabling the coating of stationary twist drills;

FIG. 2 is a sectional view taken along line 2--2 of FIG. 1;

FIG. 3 is a twist drill that has been coated with CVD diamond by thereactor disclosed herein;

FIG. 4 is an electrical block schematic representation for heating thefilament and optionally applying a bias voltage to the metal tube inwhich the twist drills are disposed;

FIG. 5 is an SEM micrograph at 80 × magnification of the head of a twistdrill with the novel reactor; and

FIG. 6 is an SEM micrograph at 300 × magnification of the drill bit ofFIG. 5.

The drawings will be described in detail in connection with thefollowing description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Twist drills are used extensively in the industrial world. The hardestand longest lasting twist drills are manufactured of tungsten carbide.Even tungsten carbide twist drills suffer premature failure whendrilling some composite materials. One means for making such twistdrills even harder is to deposit a coating of CVD diamond thereon. Thesuperb "throwing power" of the reactor design disclosed herein enablestwist drills to be coated in a stationary position with a continuousuniform film about their entire external surface including the head andflutes. This ability is surprising since CVD reactors operate using agas flow passed from an inlet to an outlet of the reactor wherein thehydrocarbon is disassociated and the disassociated mixture deposited ona substrate in the flow path of the disassociated reaction mixture.Thus, it would be expected that the twist drills would need to berotated in order for the oppositely disposed or trailing side in the gasflow to similarly be coated as the leading surface is coated.

Referring to the reactor design illustrated at FIG. 1, elongate coppertube 10 is supported at its upper and lower end by copper cooling pipe12 which is wrapped in a helical pattern about copper tube 10. The lowerend of copper tube 12, through which water or other cooling fluid flows,passes through Teflon seal 14 in table 16 which is manufactured formsuitable refractory material (e.g., molybdenum). The cooling fluid iswithdrawn about its upper end. Copper tube 10 has a plurality ofapertures in a helical parallel pattern to the cooling pipe in whichtwist drills, e.g. drill 18, are disposed radially inwardly. Whileelongate copper tube 10 preferably is cylindrical, other geometricpatterns (e.g. hexagonal, octagonal, etc.) also may be used isnecessary, desirable, or convenient. Depending upon the size of coppertube 10 and representative twist drill 18, a plurality of twist drillscan be retained for simultaneously coating with CVD diamond.

Filament 20 is suitably composed of tungsten, tantalum, or otherrefractory material. Filament 20 runs within copper tube 10 along itslengthwise extent between upper molybdenum block 22 and lower molybdenumblock 24. Again, blocks 22 and 24 can be composed of other refractorymaterial as also is appropriate for filament 20. Upper molybdenum block22 is fastened to copper tube 10 with insulating quartz pins 26 thatelectrically isolate block 22 from copper tube 10. Lower block 24 issuspended by filament 20 and constrained to vertical motion by quartzguide tube 28 which is retained in table 16 and in which the lower endfilament 20 is disposed. Filament 20 will be subject to motion duringheating and cooling cycles during reactor use and it is desired toestablish a uniform spacing between filament 20 and the twist drillsdisposed in copper tube 10.

Floating lower block 24 is connected to molybdenum block 30 by flexiblecopper braid 32 which, in turn, is connected to power line 34 that runsthrough table 16. To complete electrical connection, flexible copperbraid 36 are connected to a source of D. C. or A. C. voltage not shownbraid 36 also is connected to lower floating block 24. Power line 34 andcopper in FIG. 1 but described in connection with FIG. 4 below. Vacuumline 38 is disposed through table 16 and is connected to a source ofvacuum in conventional fashion. A metal or glass envelope is placed overthe copper tube and filament and secured against table 16 to completethe reaction chamber components. Of course, a suitable inlet foradmission of the hydrogen/hydrocarbon reaction mixture into the metal orglass envelope is provided also in conventional fashion.

Referring to FIG. 2, the radial inward placement of typical drill bit 18and the other drill bits will be observed. The spaced-apart relationshipbetween prototype twist drill 18 and filament 20 also is readilyapparent. Only the portion of twist drill 18 disposed on the inside ofcopper tube 10 is coated with CVD diamond since the hydrogen/hydrocarbonmixture passed through the inside of copper tube 10 is disassociated byfilament 20. Depending upon the size, number, spacing, etc. of the twistdrills, the distance between typical twist drill 18 and filament 20 canrange from about 2 to 20 mm. This distance is maintained constant byvirtue of quartz tube 28 that constricts movement of filament 20 in avertical direction only. The resultant CVD diamond coated twist drill isset forth at FIG. 3. Typical twist drill 18 will be seen to have acoating of CVD diamond about its head and flutes for that extent thatwas disposed within the interior of copper tube 10. The Example willamplify on such CVD diamond coated twist drill.

With respect to the process for coating of twist drills and otherelongate objects with a layer of CVD diamond, filament 20 is heated tobetween about 1750° and 2500° C. with D. C. or A. C. as can be seen byreference to FIG. 4. Voltage source 44 can supply A. C. voltage or D. C.voltage. Copper tube 10 can be grounded or it too can be subject to d.c. or a. c. bias by voltage source 42. Since filament 20 is electricallyisolated from copper tube 10 and filament 20 can be heated with D. C. orA. C., filament 20 also can be biased with respect to the twist drillswhich similarly are in electrical connection with copper tube 10 withnegative or positive D. C. voltage supplied by voltage source 44. Thevarious electrical combinations will be readily apparent to thoseskilled in the art.

With respect to conventional CVD processes usefull in the presentinvention, hydrocarbon/hydrogen gaseous mixtures are fed into a CVDreactor as an initial step. Hydrocarbon sources can include the methaneseries gases, e.g. methane, ethane, propane; unsaturated hydrocarbons,e.g. ethylene, acetylene, cyclohexene, and benzene; and the like.Methane however, is preferred. The molar ratio of hydrocarbon tohydrogen broadly ranges from about 1:10 to about 1:1,000 with about1:100 being preferred. This gaseous mixture optionally may be dilutedwith an inert gas, e.g. argon. The gaseous mixture is at least partiallydecomposed thermally hot filament 20 which normally is formed oftungsten, molybdenum, tantalum, or alloys thereof. U.S. Pat. No.4,707,384 illustrates this process.

The gaseous mixture partial decomposition also can be conducted with theassistance of d. c. discharge or radio frequency electromagneticradiation to generate a plasma, such as proposed in U.S. Pat. Nos.4,749,587, 4,767,608, and 4,830,702; and U.S. Pat. No. 4,434,188 withrespect to use of microwaves. The twist drills may be bombarded withelectrons during the CVD deposition process in accordance with U.S. Pat.No. 4,740,263.

Regardless of the particular method used in generating the partiallydecomposed gaseous mixture, the twist drills are maintained at anelevated CVD diamond-forming temperature which typically ranges fromabout 500 ° to 1100° C. and preferably in the range of about 850° to950° C. where diamond growth is at its highest rate in order to minimizegrain size. Pressures in the range of from about 0.01 to 1000 Torr,advantageously about 1-800 Torr, are taught in the art, with reducedpressure being preferred. Details on CVD process additionally can bereviewed by reference to Angus, et al., "Low-Pressure, Metastable growthof Diamond and `Diamondlike` Phases", Science, vol. 241, pages 913-921(Aug. 19, 1988); and Bachmann, et al., "Diamond Thin Films", Chemicaland Engineering News, pp. 24-39 (May 15, 1989).

The twist drills to be coated are also heated by filament 20 as is thegaseous mixture passed through copper tube 10. In order that the twistdrills not be overheated, water or other cooling medium is passedthrough cooling pipe 12 which preferably has been soldered or welded tocopper tube 10. By adjusting the temperature of cooling fluid passedthrough pipe 12 and its flow rate, the temperature of the twist drillscan be maintained within the appropriate CVD diamond-forming temperaturerange described above. The thickness of the CVD diamond layer on thetwist drills is uniform and can be as thin as 1 micrometer on up to2,000 micrometers or more, depending on processing conditions and mostimportantly upon time.

It will be appreciated that objects other than twist drills suitably canbe disposed within copper tube 10 for coating. Thus, the descriptionherein with reference to twist drills is by way of illustration and notby way of limitation. The following example shows how the presentinvention has been practiced, but should not be construed as limiting.In this application, all units and proportions are by weight, and allunits are in the metric system, unless otherwise expressly indicated.Also, all citations referred to herein are expressly incorporated hereinby reference.

EXAMPLE

In order to test the reactor configuration design, a small scale testunit was constructed utilizing a 1/4 inch thick molybdenum plate throughwhich two K-10 tungsten carbide twist drills were disposed (1 mmdiameter, Asahi Diamond Industrial Company). 5 mm of the head end ofeach of the two twist drills protruded through the copper plate and theinside of the copper plate was disposed 1 cm from a tungsten filamentthat was 9.5 in in length and 0.03 in in diameter. The tungsten filamentwas heated between 2,000° and 2,050° C. by connection to 40 amps D. C.and 31.5 D. C. volts. No filament-to-substrate bias was used although itmust be understood that a 31.5 D. C. volt applied to the filamentproduced a 31.5 volt negative bias on the filament with respect to thesubstrate at one end of the filament. At the other end of the filament,the substrate and filament were grounded together so that no biasexisted there. Hence, the average filament-to-substrate bias was about16 volts with the filament at the negative potential with respect to thesubstrate.

The reaction gas composition comprised 1.3 vol-% methane in hydrogenwith a gas flow rate of 400 SCCM. Pressure was maintained at 11 Torr.The run was conducted for 47 hours.

Optical inspection of the coated drills showed a uniform diamond coatingthat covered the head of the drills and the flutes. Even the flutesinside of the hole in the molybdenum plate were coated with diamond. Theexcellent uniform coating can be seen by reference to FIGS. 5 and 6.

We claim:
 1. In a CVD diamond reactor of a vacuum chamber, inlet forfeed hydrogen/hydrocarbon mixtures, and outlet, the improvement forcoating with CVD diamond the entire outer surface of at least a portionof a plurality of stationary elongate objects which comprises disposedwithin said reactor:(a) an elongate metal tube having a plurality ofapertures for holding elongate objects disposed readily inwardly andhaving a cooling pipe in thermal contact with and disposed about theoutside of said metal tube; and (b) a filament running within said tubealong its lengthwise extent and being in electrical connection with thesource of voltage for heating said filament to a temperature adequate toinitiate hydrocarbon disassociation, the portions of said elongateobjects within said tube surrounding said filament being heated thereby.2. The reactor of claim 1 wherein said metal tube is formed from copper.3. The reactor of claim 1 wherein said filament is electrically heated.4. The reactor of claim 3 wherein said metal tube is electricallyisolated from said filament.
 5. The reactor of claim 4 wherein a voltageis applied to said tube.
 6. The reactor of claim 1 wherein said filamentis formed of tungsten, tantalum, molybdenum, or alloys thereof.
 7. Thereactor of claim 1 wherein said filament is constrained from movementonly along its lengthwise extent.
 8. The reactor of claim 1 wherein saidcooling pipe is formed of copper.
 9. In a CVD diamond reactor comprisedof a vacuum chamber, an inlet for feeding hydrogen/hydrocarbon mixtures,and an outlet, the improvement which comprises disposed within saidreactor:(a) an elongate metal tube having a plurality of apertures forholding elongate objects disposed radially inwardly in a configurationfor coating with CVD diamond, (b) a cooling pipe in thermal contact withand disposed about the outside of said metal tube, and (c) a filamentrunning within said tube along its lengthwise extent and being inelectrical connection with a source of voltage for heating said filamentto a temperature adequate to initiate hydrocarbon disassociation and toheat portions of said elongate objects within said metal tube.
 10. A CVDdiamond reactor as in claim 9, wherein the metal tube apertures areadapted to hold the elongate objects in a configuration for coating theentire surface of a portion thereof without rotation.
 11. A CVD diamondreactor as in claim 9, wherein the metal tube apertures are adapted tohold the elongate objects in a configuration so that the surfaces to becoated are at different distances from the filament.
 12. A CVD diamondreactor as in claim 9, wherein the metal tube apertures are adapted tohold at least seven drill bits.